摘要:

SummaryVertebrates lack an epidermal nerve plexus. This feature is common to many invertebrates from which vertebrates differ by an extensive set of shared‐derived characters (synapomorphies) derived from the neural crest and epidermal neurogenic placodes. Hence, the hypothesis that the developmental precursor of the epidermal nerve plexus may be homologous to the neural crest and epidermal neurogenic placodes. This account attempts to generate a nested set of scenarios for the prevertebrate‐vertebrate transition, associating a presumed sequence of behavioural and environmental changes with the observed phenotypic ones. Toward this end, it integrates morphological, developmental, functional (physiological/behavioural) and some ecological data, as many phenotypic shifts apparently involved associated transitions in several aspects of the animals. The scenarios deal with the origin of embryonic and adult tissues and such major organs as the notochord, the CNS, gills and kidneys and propose a sequence of associated changes. Alternative scenarios are stated as the evidence often remains insufficient for decision. The analysis points to gaps in comprehension of the biology of the animals and therefore suggests further resea

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1989.tb00471.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

SummaryDasyclads (members of Order Dasycladales: Algae, Chlorophyta) are reviewed for evidence of reproductive structures in the fossil state, and then compared with what is known of the reproductive processes in living examples. The effects of poor preservation in most of the fossils are shown to result in many uncertainties, and the resulting degree of interpretation required is emphasized. Selected genera of fossil dasyclads considered relevant to this problem are briefly analysed and discussed:Archaeobatophora(Ordovician),Kulikia(Carboniferous),Imperiella(Permian),Stichoporella(Jurassic) andCympolia(Cretaceous to Recent). The pioneer views of J. Pia on dasyclad reproduction through geological time are seen to need much modification in the light of later work. The fundamental dasyclad nucleus‐fragmentation‐reproduction mechanism is believed to have operated within the great morphological variety of known dasyclads, giving rise to modification in genera where basic evolution was structural. In this way the variety of dasyclads can be seen to be the result of varied morphological evolution, often modified by the consistently simple basic reproductive mechan

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1989.tb00472.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1989.tb00473.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY